Lecture 32 Digestive secretions

Salivary Amylase

Hydrolysis of polysaccharides
-Starch and glycogen
-Mouth
-Neutral pH

Lingual Lipase

Hydrolysis of Fats
-Mouth

Mechanical breakdown

-Chewing
-increases surface area for enzymes for more efficient extraction of nutrients

Parietal cells secretions

-Stomach
-Glands in the fundus and body produce most gastric juices
-HCl
-Intrinsic factor

Hydrochloric acid HCL

-pH 1.5-3.5 resting
-Denatures proteins
-Can hydrolyze proteins (slowly)
-Breaks down plant cell walls
-kills many bacteria

Intrinsic Factor

Glycoproteins required for absorption of vitamin B12 in small intestine.

HCL Secretion: H/K pump

-Apical membrane
-H are generated form CO2 dissolving in water
-K is recycled

HCl Secretion: HCO3-Cl antiporter

-Carbonic acid breakdown generates HCO3
-HCO3 release drives Cl uptake
-blood surrounding the stomach is more alkaline

Chief Cell Secretions

Pepsinogen: inactive enzyme
-Activated to pepsin by HCL and by pepsin itself

Pepsin digestion

-Cant digest all proteins:
-Mucins (mucus)
-Intrinsic factor
- Keratin (hair ball)
Undigested polysaccharides
-Cellulose (fiber)
-Beans (polysaccharide

Duodenum

-1-2 L secreted daily in response to distension or irritation of mucosa
-Slightly alkaline (isotonic with blood plasma)
-Largely water; enzyme poor
-(Brunner's glands) secrete alkaline mucus to neutralize acidic chime

Bile

-Yellow-green; alkaline solution containing cholesterol, natural fats, phospholipids, and electrolytes. MOSTLY AMPHIPHILES

Bile salts

-Cholesterole derivatives that function in fat emulsification and absorption

Bilirubin

-Pigment formed from heme
-Bacteria break it down in the intestine to stercobilin--> brown color in feces.

Pancreatic secretions

-1,00-1,500 ml/day
-Watery alkaline solutions (pH 8) neutralizes gastric chyme

Pancreatic acinar cells

produce enzymes

Pancreatic amylase

-pH 6.7 - 7.0
-Carbohydrates

Pancreatic lipase

-pH 7.5-8.0
-fat

Pancreatic nuclease

-pH 7.0
-ribonuclease
-deoxyribonuclease

ribonuclease

breaks phosphodiester bond of RNA

Deoxyribonuclease

Breaks phosophodiester bond of DNA

Proteases

Activated in duodenum
-Trypsinogen
-Procarboxypeptidase
-CHymotrypsinogen

Trypsinogen

activated to trypsin (pH 7.5-8.5) by brush border enzyme enteropeptidase

Procarboxypeptidase and chymotrypsinogen

-pH 7-9
-Activated by trypsin

Small intestine brush border enzyme

Emzymes for breaking down
-Saccharides
-Peptides
-DNA's and RNA's

Saccharides

-Dextrinase
-Glucoamylase
-Lactase
-Maltase
-Sucrase

Dextrinase and Glucoamylase

break down oligosacchardies to monosaccharides

Lactase, Maltase, and Sucrase

break down disacchardies to monosaccharides.

NO lactase

(lactose intolerant) leads to gas and diarrhea

Peptides

-Aminopeptidases
-Carboxypeptidases
-Dipeptidases

DNA and RNA's

Nucleosidase and phosphatases

Nucleosidase and phosphatases

break down nucleic acids into free bases, pentose sugars, and phosphate ions

Digestion in small intestine

-is occurring in both the lumen and on the surface of the microvilli

Hydrolysis of fats (lipids)

-Pre-treatment: Emulsification by bile salts (does not break covalent bonds)
-Enzymes: pancreatic lipases (triglycerides) to fatty acids and monoglycerides

Hydrolysis of Nucleic Acids

-Pancreatic nucleases break down DNA and RNA into nucleotides

Phosphatase

hydrolyzes phosphate group to leave a nucleoside (sugar and base)

Nucleosidase

hydrolyze nucleoside to a sugar and base

Carbohydrate digestion

- starch and disaccharides
1. Mouth: Salivary amylase
2. Small intestine: Pancreatic amylase
- Starch and Disaccharides --> Oligosaccharides, Disaccharides, monosaccarides
3. Brush Border Enzymes in small intestine (dextrinase, glucose amylase, lactase, maltase, and sucrase.
-Turn the Oligosaccharides, Disaccharides, monosaccarides itn Lactose maltose, sucrose, galactose, glucose, and fructose.

Protein Digestion

1. Stomach: Pepsin in presence of HCl
-Large polypeptides
2. Small intestine: Pancreatic enzymes ( trypsin, chymotrpsin, carboxypeptidase)
-Small polypeptides, small peptides
3. Small intestine: Brush border enzymes (aminopeptidase, carboxypepetidase and dipeptidase)
-Amino acids (some dipeptides and tripeptides)

Fat Digestion

-Unemulsified triglycerides
1. Mouth: lingual lipase
2. Stomach: Gastric Lipase
3. Small intestine: Emulsification of the detergent action of bile salts ducted in from the liver
4. Small intestine: pancreatic lipase
-Monoglycerides or diglycerides with acidic lipases and fatty acids.

Nucleic Acid Digestion

-Nucleic acids
1. Small intestine: Pancreatic ribonuclease and deoxyribonuclease
2. Small intestine: Brush border enzyme (nucleosidase and phsophatase
-Pentose sugars, N-containing bases, and phosphate ions